| Abstract
| - On the basis of the distribution of the polymers between theseparating phases, the phase equilibria of mixturesof two polymers in a common solvent may be divided into two maincategories: In an associative phaseseparation, both polymers are enriched in one of the separating phases,and in a segregative phase separation,the two polymers are enriched in separate phases. This workinvestigates the conditions for the two types ofphase separation, as predicted by the Flory−Huggins theory. Theemphasis is on the (previously largelyneglected) associative phase separation and on transitions fromsegregative to associative phase behavior(S−A transitions) through changes in the pair interaction parameters.An associative phase separation isalways favored by marginal or poor solvent conditions and may sometimesoccur even without an effectivepolymer−polymer attraction. An S−A transition may be generatedby changing the polymer−polymerinteraction but also, under certain circumstances, by changing only oneof the polymer−solvent interactionparameters. When one of the polymers is only partly miscible withthe solvent, a continuous S−A transitionmay occur, where the same two-phase area gradually evolves from beingsegregative to being associative. Acontinuous S−A transition always occurs via a region ofborderline phase separation, where one of thepolymersdistributes equally between the two separating phases. Theborderline conditions are also conditions formaximum miscibility (a minimum extension of the two-phase area) in thesystem.
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